Conventionally, zoom lens systems of a two-unit construction consisting of a negative and a positive lens unit and thus having a retrofocus-type power distribution have been widely used as interchangeable lenses for single-lens reflex cameras that are generally considered to require comparatively large back focal lengths. In addition, zoom lens systems of this type, with which it is easy to secure a large back focal length, are useful also as comparatively low-cost, low-zoom-ratio zoom lens systems for use in electronic still cameras that have recently been coming into wider and wider use, because such cameras also require sufficiently large back focal lengths to arrange a low-pass filter and others between a taking lens and an image-sensing device (such as CCD).
In electronic still cameras, for which every effort is constantly being made to make the image-sensing device ever more compact, the problem to be solved most urgently is that of achieving the miniaturization and cost reduction of the taking lens without a loss in image quality. As one solution to this problem, Japanese Laid-open Patent Application No. H4-46308 proposes a zoom lens system of a two-unit construction consisting of a negative and a positive lens unit in which miniaturization and cost reduction are attempted by extremely reducing the number of the constituent lens elements. This zoom lens system, consisting of a front lens unit of a two-unit/two-element construction having a negative optical power and a rear lens unit of a two-unit/two-element or three-unit/three-element construction having a positive optical power, provides a zoom ratio of about 2 .times..
However, when applied to a small image-sensing device such as is used in an electronic still camera, the zoom lens system proposed in Japanese Laid-open Patient Application No. H4-46308, exactly because it consists of an extremely small number of lens elements, requires too strong an optical power in each lens element. As a result, the distance of the air space between the first and second lens elements of the rear lens unit becomes so sensitive to errors from spherical aberrations, especially at the telephoto end, that it is extremely difficult to shape those lens elements with sufficiently high accuracy.
Moreover, as the solid-state image-sensing device is made smaller, the taking optical system as a whole should ideally be made accordingly smaller. However, in any unit that includes an optical system, it is difficult to miniaturize components such as lens barrels that are not purely optical components with the same reduction factor as purely optical components.
Furthermore, there are various problems associated with a zoom lens system for use in a camera fitted with a solid-state image-sensing device such as a CCD. For example, it is difficult to maximize the distance at which the exit pupil is placed (i.e. to minimize the incident angle with which peripheral rays enter the CCD) and at the same time minimize the total length of the zoom lens system; it is also difficult to obtain acceptable aberration characteristics over the entire zoom range and at the same time secure sufficient peripheral illumination at the wide-angle end.